1. Field of the Invention
This invention relates to a papermaking machine press. More particularly, this invention relates to a papermaking machine press having a wide area of pressing contact. Still more particularly, this invention relates to a papermaking machine press having a wide area of pressing contact wherein the profile of the nip load, extending in the machine direction, can be selectively controlled. Even still more particularly, this invention relates to a wide area, or extended nip, type of papermaking machine press which utilizes a shoe which is pivotally mounted about two, parallel axes.
2. Description of the Prior Art
A typical example of the best prior configuration of a so-called extended nip type of papermaking machine press includes a backing roll having a smooth, continuous support surface and a shoe having a curved, convave face surface. The radius of curvature of the shoe is slightly larger than the radius of curvature of the backing roll surface. The paper web to be dewatered is passed through the wide, or extended, nip between the shoe surface and the backing roll with at least one felt on one side of the web and, usually, a second felt on the other side of the web. Intermediate the surface of the shoe and any felt on the shoe side of the paper web, or the paper web in the case where there is no felt on the paper web side of the shoe, is a belt which presents a co-traveling first surface with the felt, or web, and a second surface which engages the shoe in sliding contact.
The shoe is hydraulically actuated to provide pressure over a wide area between the shoe, the belt and paper web traveling over the backing roll surface. In order to attain a condition of equilibrium between the surface of the pressure shoe against the dynamic forces of the belt, felt(s) and web over the surface of the backing roll, the pressure shoe is pivoted about a cylindrical rod, such as shown and described in U. S. Pat. No. 4,425,190 (Cronin), which co-extends parallel with the rotational axis of the backing roll longitudinally in the cross-machine direction. This allows the shoe to come into an equilibrium condition wherein the hydrodynamic forces acting on the paper web, felts and belt, including any lubricant passing through the nip between the belt and shoe, are balanced about the cylindrical rod supporting the pressure shoe.
Such an arrangement operates well and provides paper web dewatering over a wide area pressing zone which substantially exceeds the narrow contact area to which the paper web is exposed in an ordinary nip between co-rotating opposed rolls in a press couple in a papermaking machine. However, it is desired that the water removal process in the pressing operation of a papermaking machine should be optimized, in conjunction with optimized paper formation, to obtain a finished paper product having superior qualities, and combinations of qualities, relating to such parameters as, for example, burst strength, tensile strength, texture, surface smoothness, fiber distribution and bulk. Such optimization is a function of machine speed, nip load, and the pressure profile, all in conjunction with the grade of paper being produced.
Prior shoe-types of extended nip presses have limited operational flexibility due to the rigid geometry of the shoe and its range of movement, or adjustability, relative to the backing roll when in operation. Since the shoe has a single pivot, once the papermaking machine reaches a given speed, the hydrodynamic forces acting on the shoe locate it in a position relative to the backing roll which is determined by these forces. This position, in turn, determines the dewatering pressure profile acting on the paper web. The shoe is maintained in this position until some combination of the machine speed or nip loading force against the shoe over the backing roll changes. At a certain machine speed, at a certain nip load, the pressure profile will be optimal, or nearly optimal, for producing a certain grade of paper. However, either increasing the machine speed, or increasing the total nip load, or some combination of both, does not result in a pressure profile which necessarily remains optimal for the grade of paper being produced. Further, there is nothing which can be done to remedy the non-optimal pressure profile when the machine speed and/or nip loading changes since the shoe is positioned solely by the hydrodynamic forces of the machine components passing through the extended nip while the shoe remains on its fixed pivot.
Some shoe-type extended nip presses have been made wherein the shoe has a plurality of parallel grooves extending longitudinally such that the shoe can be pivotally mounted over a pivot rod which can be positioned in a selected one of such grooves. Such a configuration is shown and described in U.S. Pat. No. 4,973,384 (Cronin). This allows the pressure profile in the extended nip to be changed, but to change the location of the pivot rod in a selected groove of the shoe requires that the papermaking machine be shut down for a considerable period of time to effect this change. This not only is costly, due to lost production time, but the pressure profile for each groove is also fixed so that operation of the extended nip press is only optimal for a given grade of paper for a certain combination of machine speed and nip load within a relatively narrow range regardless of which pivot groove is selected.
As a result of these facts relating to the physics involved in the operation of such a wide area, extended nip type of papermaking press having a pressure profile, each such press essentially has to be designed to manufacture a specific paper product at a specific nip pressure at a specific operating speed, all within a relatively narrow range of the parameters involved.